Floor-covering materials, such as carpet tile, can contribute to the buildup of high static electrical charges, as a result of the motion of people to walking or to the movement of furniture or other objects in contact with the floor-covering materials. Abrupt discharges of such accumulated static charges on the floor-covering material may cause discomfort to personnel, and also may cause malfunction of computer or electronic equipment. Static buildup and discharge may be minimized by providing a conductive path from the face surface of the carpet to the electrical ground, and assuring that the resistance for the floor surface material; that is, the bulk resistance and surface resistivity, is within defined standards, such as those standards required by computer and electronic manufacturers, for the protection of sensitive computer, electronic and microchip-containing equipment. Typically, such standards require a minimum of 5.times.10.sup.5 ohms and a maximum of 2.times.10.sup.10 ohms in surface and bulk resistivity. In addition, there is often a desirable minimum resistivity for safety of 10.sup.3 to 10.sup.4 ohms.
Carpet materials, particularly carpet tile, have been developed to prevent the formation of static charges or the accumulation of static charges and which permit the rapid dissipation of such charges without danger to personnel or equipment; however, to date such floor-covering materials have not been wholly satisfactory. Antistatic floor-surface materials may be produced by applying an antistatic topical agent to the face of the floor covering; however, such antistatic treatments typically are not permanent and can contribute to soiling characteristics of the floor covering, and further usually are not very effective in low humidity and relative humidity conditions. Permanent electrical conductivity or electrical static conductivity in a fibrous floor coverings may be produced by incorporating conductive fiber or yarn elements into the face of the floor covering, such as the carpet tile. Such conductive fibrous elements may comprise, for example, fine, thin, metal wires or metallic-coated wires as yarn elements, or be composed of carbon-containing, such as carbon-filled, core yarns or fibers incorporated into the nonconductive face fibers or yarns. Such permanent protection often provides both low static generation from the interaction between the face of the carpet and people and objects, and also electrical conductivity across the face of the carpet. However, electrically conductive fiber faces, by themselves, do not insure efficient electrical conductivity through the face of the carpet; that is, acceptable transmission or bulk resistivity (also called volume resistivity).
Carpet tile has been produced wherein static charges generated are dissipated very rapidly through the employment of electrically conductive fiber or yarn elements incorporated in the fibrous face of the carpet and anchoring the fibrous face of the carpet and these elements to a primary backing, through the employment of an electrically conductive precoat or backing layer. Typically, such electrically conductive precoat comprises a polymeric material, such as a styrene-butadiene latex incorporating therein electrically conductive agents, such as metallic particles or more particularly dispersed finely-divided carbon-black particles, in order to provide for increased electrical conductivity. Such carpet tile and other carpet material also require a relatively thick backing layer, to insure dimensional stability of the carpet tile and the laying flat of the carpet tile in use. A wide variety of polymeric materials may be used; however, atactic polypropylene, bitumen and polyvinyl chloride resin are presently the most popular materials used in the backing of commercial carpet tile. Such materials are essentially nonelectrically conductive and, therefore, do not permit an easy pathway for electrical charges to the ground or to the back of the carpet tile.
Generally, the employment of antistatic or electrically conductive agents into the relatively thick backing layer, such as the employment of particulate particles, such as carbon-black particles, with a polymeric backing material, has not proven to be satisfactory, since the amount of carbon black required for the desired electrical conductivity or antistatic effect provides for a significant increase in viscosity and often otherwise affects the chemical properties of the backing. Also, the employment of other antistatic additives, such as, for example, metal fatty-acid soaps, glycols and the like, where incorporated into the backing material, often modifies and changes the chemical and mechanical properties of the backing, resulting in unsatisfactory specifications or properties. For example, the employment of stearates within a solid bitumen backing, employed for the preparation of carpet tile, often affects the adhesive qualities of the bitumen and further weakens the mechanical properties of the bitumen or bitumen-filler; for example, limestone, backing material. Carpet tiles and a method of manufacturing carpet tiles are set forth in British Patent No. 2057353B, published Mar. 28, 1984, hereby incorporated by reference.
Thus, it is desirable to provide a more effective and improved antistatic, electrically conductive, fibrous-face, surface-coating material, particularly a carpet tile, with improved electrical conductivity, and which material avoids the disadvantages associated with the prior-art surface materials and methods of production.